1. Field of the Invention
The present invention relates to the use of nonlinear devices for the conversion of optical and near optical frequencies to higher or lower values. Such devices include second harmonic generators and parametric amplifiers. Specifically, deuterated l-arginine phosphate crystals in a particular crystallographic orientation with incident light radiation will perform this conversion independently of temperature at a preferred frequency of 1.064 .mu.m with high efficiency.
2. Description of Related Art
The application of lasers to research and applied projects in 1960 was limited by the number of frequencies at which the lasers operated. Nonlinear devices were the first major advance in enlarging the scope of operating frequencies. Nonlinear devices can convert a given frequency of electromagnetic radiation to a higher value by creating a radiation field at the new frequency, or can convert two frequencies to their sum or difference value. Although nonlinear devices significantly broadened the spectrum of frequencies of laser operation, the majority of the optical spectrum was still unavailable.
The development of dye lasers opened up the entire visible spectrum and a significant portion of the infrared and ultraviolet regions for laser operation. Nonlinear devices have advantages over dye lasers, however, which account for their continued use. Nonlinear devices are simpler and cleaner to operate than dye lasers, and more rugged for applied use. In many regions of the spectrum, nonlinear devices are more efficient than dye lasers and can convert readily available 1.064 .mu.m light by stepwise doubling to 532 nm and then to 266 nm.
Optical harmonic generation, first observed in 1961, has been widely used to further extend the operational wavelengths of lasers, and nonlinear devices are commonly used as optical harmonic generators. The discovery of second harmonic generation was made by Franken et al. and described in "Generation of Optical Harmonics", Phys. Rev. Lett., Vol. 7, No. 4, pp. 118-119 (Aug. 15, 1961). Franken focused a 694.3 nm light beam from a ruby laser onto a quartz crystal and found that some of the light was converted to second harmonic light, which is exactly half the wavelength (347.15 nm), with photons being twice the energy of the incident light.
The second harmonic is generated as incident light interacts with the electrons in a nonlinear material, such as quartz. In nonlinear crystals, incident light travels at different speeds for different directions of vibration within the crystal, and therefore the optical properties of the crystal (e.g., dielectric constant) can be different along each of three orthogonal crystal axes. Conversely, light travels with the same speed, regardless of its direction of vibration, within isotropic media.
The light from a point source in an anisotropic crystal spreads out in two wave surfaces, an ordinary ray and an extraordinary ray, with one traveling faster than the other. When the extraordinary ray travels faster than the ordinary ray in a uniaxial crystal, the crystal is considered negative (-); when the extraordinary ray travels slower, the crystal is considered positive (+).
The two resulting rays of light generated by the nonlinear material are at the incident frequency (fundamental harmonic motion), and at twice the incident frequency (second harmonic motion). The strongest interaction of light beams occurs when the phase velocity of the two waves are the same, or phase-matched. The efficiency of the second harmonic generation (SHG) process is a function of the phase mismatch between the incident wave and the generated second harmonic wave, and highly efficient frequency conversion of intense laser radiation demands careful attention to the problem of phase mismatching.
The production efficiency of the second harmonic is also limited by the natural dispersion of the nonlinear material, which causes different wavelenghts of light to travel at different speeds. Dispersion causes the frequency-doubled light in the nonlinear material to travel more slowly than the incident radiation, resulting in the incident light being out of phase with the slower, frequency-doubled light. The two waves of light destructively interfere, and the intensity of frequency-doubled light leaving the crystal is severely diminished. Dispersion in the refractive indices of all materials usually prevents phase-matching from occurring unless special steps are taken.
The application of nonlinear phenomena to practical devices was limited because of phase mismatching until a discovery by Giormaine and Kleinman (U.S. Pat. No. 3,234,475). They used the birefringence of an optically anisotropic crystal to offset the phase mismatch caused by dispersion. Today, the most common method of achieving phase-matching in crystals is by using birefringent crystals.
In a birefringent material, an incident beam of light is split into two waves traveling at different velocities. In a birefringent negative uniaxial crystal, the extraordinary ray travels faster than the ordinary ray. While dispersion lowers the velocity of the frequency-doubled light with respect to the incident light, birefringence has the opposite effect. If the ordinary ray is at the incident light frequency, and the extraordinary ray is the frequency-doubled light, then both dispersion and birefringence act on the frequency-doubled light, and the two effects can cancel each other out.
The magnitude of birefringence depends on the angle of incidence (the angle between the incident beam and the optic axis of the crystal), and therefore an angle can be chosen for which the birefringence is sufficiently large to compensate for the effects of dispersion. When phase-matching is accomplished, the power of the generated wave is many orders of magnitude greater than in non-phase-matched interactions, and the production efficiency of the frequency-doubled light rises dramatically. The phase-matching conditions are achieved by choosing the proper temperature and propagation direction, and depend on the wavelength of the incident radiation. This technique of angle tuning the birefringence achieves minimum angular sensitivity at a noncritical phase-matching angle of incidence of 90.degree..
Although all sources of phase mismatch should be controlled, the relative importance of each source is determined by the application. The thermal contributions to phase mismatch are especially crucial with harmonic generation of high average power laser radiation. Small amounts of absorption of the fundamental or harmonic wavelengths lead to large thermal fluctuations, thermal gradients, and thermal stress, all of which cause variations in the refractive index. The propagation direction required to obtain phase-matching is highly dependent on the refractive indices, and therefore the thermal effects in the nonlinear crystal must be carefully controlled to minimize phase mismatch and maintain efficient frequency conversion. Various schemes such as electroptic tuning, piezo-optic tuning, beam shaping, and gas-cooled multiple plate designs have been proposed to control thermal loads and compensate for thermal effects in nonlinear optical crystals used for high average power harmonic generation.
A technique for controlling the birefringence-dispersion relationship by exploiting the temperature dependence of phase-matching is described in U.S. Pat. No. 3,262,658 by Ballman et al. (1966). The invention attains noncritical phase-matching conditions and controls the magnitude of birefringence by operating at a fixed temperature. The temperature must be closely controlled, since small variations will detune the crystal.
The number of effective and efficient nonlinear materials for second harmonic generation, which take advantage of the temperature tuning method, remains limited. One example of a crystal that performs stepwise frequency doubling is described in U.S. Pat. No. 3,965,375 by Bergman et al. (1976). A lithium perchlorate trihydrate (LiClO.sub.4.3H.sub.2 O) crystal is employed to achieve noncritical phase-matching at about 530 nm and at room temperature, and is insensitive to temperature variations.
The use of thermal management techniques can be obviated by the use of nonlinear crystals that possess temperature-insensitive, phase-matching directions. An ideal crystal would have a large nonlinear coupling in a phase-matching direction that has a zero-valued rate of change with respect to temperature for the incident frequency.
L-arginine phosphate and deuterated l-arginine phosphate (dLAP) are relatively recent additions to the set of materials known to provide efficient frequency conversion in the near infrared and visible regions of the electromagnetic spectrum. The dLAP crystal was patented in U.S. Pat. No. 4,697,100 by Eimerl (Sep. 29, 1987) as a new composition of matter for application in harmonic conversion of laser light, especially in the one micron wavelength region. The measurement and calculation of unique temperature-insensitive, phase-matching directions in these crystals are presented in the following invention description.